Hardening reaction

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Hardening reaction (heparin test) - a characteristic reaction that allows the detection of sulfates and other sulfur compounds in sparingly soluble samples, for which precipitation reactions can not be used. The name is derived from the reddish-brown coloration of the liver produced by the sulfur melting with alkali carbonates (hepar sulfuris).

Sulphate roasted on charcoal with Na2CO3 is reduced to water-soluble Na2S:

BaSO4 + Na2CO3 + 4C → BaCO3 + 4CO↑ + Na2S

Sodium sulfide is also formed for other sulfur-containing compounds. The resulting alloy, moistened with water on a silver plate, leaves a dark brown or black spot due to the formation of black Ag2S:

2Na2S + 4Ag + 2H2O + O2 → 4NaOH + 2Ag2S

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